The ATLAS3D project – XVII. Linking photometric and kinematic signatures of stellar discs in early-type galaxies

Abstract

We analyse the morphological structures in galaxies of the ATLAS 3D sample by fitting a single Sérsic profile and decomposing all non-barred objects (180 of 260 objects) in two components parameterised by an exponential and a general Sérsic function. The aim of this analysis is to look for signatures of discs in light distributions of nearby early-type galaxies and compare them to kinematic properties. Using Sérsic index from single component fits for a distinction between slow and fast rotators, or even late-and early-type galaxies, is not recommended. Assuming that objects with n > 3 are slow rotators (or ellipticals), there is only a 22 per cent probability to correctly classify objects as slow rotators (or 37 per cent of previously classified as ellipticals). We show that exponential sub-components, as well as light profiles fitted with only a single component of a low Sérsic index, can be linked with the kinematic evidence for discs in early-type galaxies. The median disk-to-total light ratio for fast and slow rotators is 0.41 and 0.0, respectively. Similarly, the median Sérsic indices of the bulge (general Sérsic component) are 1.7 and 4.8 for fast and slow rotators, respectively. Overall, discs or disclike structures, are present in 83 per cent of early-type galaxies which do not have bars, and they show a full range of disk-to-total light ratios. Discs in early-type galaxies contribute with about 40 per cent to the total mass of the analysed (non-barred) objects. The decomposition into discs and bulges can be used as a rough approximation for the separation between fast and slow rotators, but it is not a substitute, as there is only a 59 per cent probability to correctly recognise slow rotators. We find trends between the angular momentum and the disc-to-total light ratios and the Sérsic index of the bulge, in the sense that high angular momentum galaxies have large disc-to-total light ratios and small bulge indices, but there is none between the angular momentum and the global Sérsic index. We investigate the inclination effects on the decomposition results and confirm that strong exponential profiles can be distinguished even at low inclinations, but medium size discs are difficult to quantify using photometry alone at inclinations lower than ∼ 50 • . Kinematics (i.e. projected angular momentum) remains the best approach to mitigate the influence of the inclination effects. We also find weak trends with mass and environmental density, where disc dominated galaxies are typically less massive and found at all densities, including the densest region sampled by the ATLAS 3D sample.

MegaMorph – multiwavelength measurement of galaxy structure: physically meaningful bulge–disc decomposition of galaxies near and far

“…As before, we include galaxies with elliptical morphologies in these plots for two reasons. First, recent work has blurred the lines between elliptical and lenticular galaxies, with many ellipticals found to contain faint disc components when studied in detail (Krajnović et al 2013). Second, our aim is to inform work on large surveys, which may not have morphological classifications available.…”

“…We note that almost all the early-type galaxies with low (b/a) disc are fast rotators, including both of the elliptical galaxies which fall in this region of parameter space dominated by S0s. The early-types with (b/a) disc > 0.5 are a mixture of fast and slow rotators, however most (or all with n b > 2) have been classified as elliptical galaxies (see Cappellari et al 2011 andKrajnović et al 2013 for a more thorough study of this topic).…”

Star formation in the outer regions of the early-type galaxy NGC 4203

“…Based on the total stellar mass reported by Cappellari et al (2013) and on the B/D ratio given in Krajnović et al (2013), the mass of the stellar disc is ∼2.7 ×10 10 M . At the current SFR, this mass would grow by a mere 0.1 percent in 1 Gyr.…”

“…NGC 4203 has been observed at many wavelengths. Optical studies show that NGC 4203 has a relatively massive stellar disc (B/D = 0.4-0.5 -Burstein 1979; Krajnović et al 2013). Early H i observations revealed a low-column-density H i disc with a complex morphology (Burstein & Krumm 1981;van Driel et al 1988).…”

“…Recently, a number of studies based on broad-band imaging and integral-field spectroscopy at optical wavelengths have concluded that most ETGs host a stellar disc, and that ETGs as a family cover a similar range of B/D ratio as spirals (Cappellari et al 2011;Laurikainen et al 2011;Kormendy & Bender 2012;Krajnović et al 2013;Weijmans et al 2014). Discs are detected also with a number of additional techniques such as narrow-band imaging of emission lines (e.g., Hβ), imaging of dust absorption, or the observation E-mail: mkyildiz@astro.rug.nl of the kinematics of planetary nebulae and globular clusters (Romanowsky et al 2003;Sarzi et al 2006;Pota et al 2013).…”